Associative plasticity in surround inhibition circuits in human motor cortex

Surround inhibition is a physiological mechanism that is hypothesised to improve contrast between signals in the central nervous system. In the human motor system, motor surround inhibition (m SI ) can be assessed using transcranial magnetic stimulation ( TMS ). We evaluated whether it is possible to modulate m SI , using a paradigm able to induce plastic effects in primary motor cortex ( M 1). Fifteen healthy volunteers participated in the experiments. To assess m SI , we delivered single pulses at rest and at the onset of a right thumb abduction. TMS pulses over abductor digiti minimi ( ADM ; surround muscle) hotspot were delivered when EMG activity in right abductor pollicis brevis ( APB ; active muscle) > 100 μV was detected. Paired associative stimulation ( PAS ) was delivered using peripheral median nerve electric stimulation and TMS over APB M 1 area at an interstimulus interval of 21.5 ms for the real PAS ( PAS 21.5) and 100 ms for the sham PAS ( PAS 100). To verify the effect of PAS 21.5 on m SI we collected 20 MEP s from ADM at rest and during APB movements before ( T 0) and 5 ( T 1), 15 ( T 2) and 30 ( T 3) minutes after PAS 21.5. mSI from APB to ADM was present at baseline. PAS 21.5 increased the amount of m SI compared with baseline whereas there was no effect after PAS 100. Our results suggest that m SI is an adaptable phenomenon depending on prior experience.